Sen. John McCain picked up an endorsement Tuesday from a past rival, former Massachusetts Gov. Mitt Romney.

(CNN) - Mitt Romney endorsed Sen. John McCain, one of the former Massachussetts governor's main competitors for the 2008 Republican presidential nomination, for re-election to the U.S. Senate on Tuesday.

"Senator McCain's record of service and sacrifice for America is honored by all," Romney said in a statement. "But I believe that it is his core values of courage, faith and honor – forged in battle and confirmed by a lifetime of service to America – that make Senator McCain's leadership in the United States Senate so necessary in these perilous times."

Romney said it is "hard to imagine the United States Senate without John McCain."

McCain faced off with Romney in 16 Republican primary debates but bowed out of the primary race after the Super Tuesday contests.

McCain faces a primary challenge from former Rep. J.D. Hayworth.

Over the past few weeks, McCain has announced endorsements from top conservatives like Dick Armey of FreedomWorks and Grover Norquist, president of Americans for Tax Reform. Last month, McCain's 2008 running mate, former Alaska Governor Sarah Palin, announced that she would travel to Arizona to campaign with McCain in late March. Palin's popularity with voters on the right could help McCain with some Republican voters who question his commitment to conservative values and goals.

Sen. Scott Brown, whose victory in last month's special senate election in Massachusetts energized the GOP, has also said he'll stump with McCain early next month. And Arizona's Republican congressional delegation announced last week that it will support the four-term senator.